1. Field of the Invention
The present invention relates to an electron source having a needle cathode, and an electron beam apparatus using such an electron source, such as a scanning electron microscope, a scanning transmission electron microscope, an Auger electron spectroscope, an electron beam lithography apparatus or a wafer inspection apparatus.
2. Description of the Related Art
In recent years, electron sources using cathodes with monocrystalline tungsten needle electrodes having coating layers of zirconium and oxygen (hereinafter referred to as ZrO/W electron sources) have been used to obtain electron beams that are brighter and have a longer operating life than thermionic cathodes (see Non-Patent Document 1).
ZrO/W electron sources are obtained by providing a reservoir consisting of zirconium and oxygen on a needle cathode composed of tungsten monocrystals having an axial orientation in the <100> orientation, so that the zirconium and oxygen are diffused to form a coating layer (hereinafter referred to as a ZrO coating layer), said ZrO coating layer reducing the work function of the (100) plane of the tungsten monocrystals from 4.5 eV to about 2.8 eV, so that only the minuscule crystalline facet corresponding to the (100) plane formed at the tip of this cathode forms an electron emission region, as a result of which an electron beam that is brighter than that of conventional thermionic cathodes can be obtained, and the operating life is also prolonged. Techniques of appropriately processing the shapes of the cathode tips are also known as methods for stabilizing the supply of electrons in the aforementioned cathodes (see Patent Document 1).
Additionally, techniques of forming the aforementioned coating layers of barium and oxygen are also known (see Patent Document 2, Patent Document 3 and Patent Document 4).
As shown in FIG. 7, a ZrO/W electron source comprises a needle cathode 1 of tungsten in the <100> orientation for emitting an electron beam attached by welding or the like to a portion of a tungsten filament 3 provided on a conductive terminal 4 anchored to insulator 5. A reservoir 2 of zirconium and oxygen is formed in a portion of the cathode 1. While not shown in the drawings, the surface of the cathode 1 is covered with a ZrO coating layer.
The tip portion of the cathode 1 of the ZrO/W electron source is positioned between a suppressor electrode and an extractor electrode. The cathode 1 is electrically heated to about 1800 K by means of the filament 3, and generally, a high negative voltage is applied between the extractor electrode and the cathode 1, while a negative voltage of a few hundred volts is applied between the cathode 1 and the suppressor electrode, to suppress thermal electrons emitting from the filament 3.
While ZrO/W electron sources are widely used in critical dimension SEM and wafer inspection equipment at low accelerating voltages, when observing SEM images at relatively high magnifications, vibrations from around the equipment can generate noise, thus reducing the resolution, sometimes to such a degree that measurements are not possible. The filament 3 of a ZrO/W electron source tends to vibrate in a direction perpendicular to the plane formed by three points, these being the cathode 1 and the two portions where it connects to the conductive terminals 4, and the vibrations will have a characteristic frequency. It is known that when the frequency of external vibrations matches with this characteristic frequency, the filament can resonate, thus generating noise.
On the other hand, while differing completely from the present invention in terms of the field and applications, degradation of properties due to the generation of noise by minute vibrations is not considered to be problematic in electron sources having planar cathodes with large electron emission regions such as those used in cathode ray tubes for displaying images (see Patent Document 5 and Patent Document 6).
Additionally, in the case of electron sources comprising planar cathodes with large electron emission regions of this kind, wide filaments are used to make it easier to mount the cathode. Generally, the operating temperature of these electron sources is rather low, about 1000 K, and heat can be maintained with relatively little current even when a wide filament is used.    Non-Patent Document 1: D. Tuggle, J. Vac. Sci. Technol., 16, p. 1699 (1979).    Patent Document 1: JP 2005-339922 A    Patent Document 2: JP 2004-265614 A    Patent Document 3: JP 2005-332677 A    Patent Document 4: JP 2005-222945 A    Patent Document 5: JP S55-148339 A    Patent Document 6: Japanese Utility Model Application, Publication No. S48-27623